I was just thinking about fine-tuning, and it occured to me that there are two common objections to fine-tuning that seem to be in conflict. Maybe they're not. It just now occurred to me, so this post is off the cuff.
The first objection is that if the universe were fine-tuned for life, we should expect to see more of it in the universe. But the reality of the matter is that the vast majority of the universe is hostile to life.
The second objection is that the universe is only fined-tuned for life as we know it. There may be other kinds of life we haven't thought of, like silicon based life instead of carbon based life.
If the second objection is sound, it would lead you to believe that there are a wide range of conditions under which there could be life. But the first objection tells you that there isn't. So they can't both be sound objections.
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I tend to make the more modest argument that physics is fine tuned for life in the sense that life needs stable and habitable places like stars and planets, and also depends upon certain properties of chemistry to exist. A universe offering a broad range of available elements, many of which have unique properties that permit diverse assembly and metabolic function, is advantageous to any type of life one wishes to imagine. It is a fact, though, that carbon based life offers unique advantages that are not true with other elements, like silicone (which is sometimes proposed). There is fine tuning involved in having such an element, as well as having it (and some others) in the kind of abundance you would hope for.
Maybe it would better to say, "fine tuned for chemistry," or "fine-tuned for complex chemistry," or "fine-tuned for complexity."
The atheist cosmologist Martin Rees describes it as "fine tuned for complexity." One of the most fundamental hurdles is the fact that differences in such things as the global cosmic energy density, density fluctuations in the early universe, and the cosmological constant all would impact whether the universe grew in such a way that galaxies and stars formed at all (it could all be just diffuse hydrogen gas), or that it simply collapsed again before anything "interesting" could occur.
Unless there is an abundance of life and we haven't detected most of it because we don't know what to look for? In that case objection 1 is mistaken. It isn't a very convincing objection anyway, for all we know this universe has a probable density of life given the laws of physics and probability. This objection would be countered by any model with different constants producing a more life-dense outcome. Good luck with that!
I don't find fine tuning arguments convincing, they smack of 'puddle thinking'.
"Life density" may have to be calibrated, too. What counts as "biologically dense?" I mean strictly speaking if there were life on every single planet in the universe, but nowhere else, a person could rightly say that life isn't very dense because there's so much empty space. But that seems wrong.
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